The present invention relates to a display device and, more particularly, to an active-matrix type display device wherein an active element includes a semiconductor film which is formed by modifying properties of a semiconductor film formed on an insulating substrate with laser light and a method for manufacturing the display device. Hereinafter, the display device may sometimes be referred to as “display apparatus” or simply as “display”.
The active-matrix type display device (also referred to as “display device with an active matrix type driving system” or “display apparatus”), which includes an active element such as a thin film transistor or the like as a driving element for each of pixels arranged in a matrix, has widely been used. As is known to those skilled in the art, many kinds of such active-matrix type display device can display high quality images owing to a plurality of pixel circuits arranged on a substrate, each of the pixel circuits being composed of the active element such as the thin film transistor formed by using a silicon film as the semiconductor film. Hereinafter, the thin film transistor, which is a typical example of the active element, will be described.
It has been difficult to form a high-speed and high-performance circuit with the use of the thin film transistor formed by using an amorphous silicon semiconductor film (hereinafter also referred to as “silicon semiconductor film” or “silicon film”), which has typically been used as the semiconductor film, due to limitations of properties, such as a mobility, of the thin film transistor. In order to realize a high mobility thin film transistor which is required for providing a better image quality, it is effective to form the thin film transistor by using a crystallized polysilicon film prepared by modifying (crystallizing) the amorphous silicon film in advance.
The modification (crystallization) of the amorphous silicon film into the polysilicon film and improvements in crystallinity have been achieved by excimer laser light (also referred to as “laser beam” or simply as “laser light”) irradiation. Such crystallization method is described in detail in, for example, non-patent documents 1 to 3 and so forth.
The modification of amorphous silicon film through the crystallization employing the excimer laser light irradiation will be described with reference to FIGS. 26A and 26B. FIGS. 26A and 26B illustrate an excimer pulsed laser light scanning which is the most popular crystallization method. A structure of a glass substrate, on which a semiconductor layer to be irradiated with the laser light is formed, is shown in FIG. 26A, and a state of the modification achieved by the laser light irradiation is shown in FIG. 26B. The substrate is typically formed from glass or ceramic, and an example wherein the glass substrate is used will be described below. An amorphous silicon film 302 which is deposited on the glass substrate 301 with an undercoating layer (SiN layer or the like; not shown) interposed therebetween is irradiated with a linear excimer laser beam 303 having a width of several millimeters to several hundreds of millimeters. Scanning is then performed wherein the irradiation positions are changed by 1 to several pulses along one direction (x direction) indicated by an arrow, to thereby modify the amorphous silicon film 302 covering the whole substrate 301 into a polysilicon film 304. The polysilicon film 304 modified by the above-described method is subjected to various processing such as etching, formation of wiring, and ion implantation to obtain an active-matrix substrate on which thin film transistor circuits for driving is formed for each of pixel portions. The thus obtained substrate is used for manufacturing an active-matrix type display such as a liquid crystal display, and an organic EL display.
FIG. 27A is a plan view showing a part of the laser light irradiation portion shown in FIG. 26B, and FIG. 27B is a plan view showing a configuration of a main part of a thin film transistor portion of FIG. 26B. As shown in FIG. 27A, many crystallized silicon grains each having a grain size of about 0.05 μm to 0.5 μm grow uniformly on a surface of the laser light irradiation portion. Each of boundaries of the silicon grains (i.e., silicon crystals) is closed. The portion enclosed in a rectangle in FIG. 27A is a transistor portion TRA which is to be used as the semiconductor film of each of the thin film transistors. The above-described crystallization represents the conventional silicon film modification, and it should be emphasized that the modification of the present invention is different from the conventional technique.
In order to form a pixel circuit by using the modified silicon film 304, an MIS transistor is manufactured by: removing an unnecessary portion, which is a portion other than that to be used as the transistor TRA shown in FIG. 27A, so as to use a portion of the crystallized silicon as the transistor portion as shown in FIG. 27B; forming an island shaped silicon film; and then arranging a gate insulating film (not shown), a gate electrode (GT), a source electrode (SD1), and a drain electrode SD2 on the thus obtained island PSI. This transistor formation technique is known to those skilled in the art. Since the modification operation has been performed on all parts of the pixel portion in the conventional technique, the conventional technique has been insufficient in efficiency of the modification.
The following represents background literature.
Non-Patent Literature I
T. C. Angelis et al.; Effect of Excimer Laser Annealing on the Structural and Electrical Properties of Polycrystalline Silicon Thin-Film Transistor, J. Appl. Phy., Vol. 86, pp 4600–4606, 1999.
Non-Patent Literature 2
H. Kuriyama et al.; Lateral Grain Growth of Poly-Si Films with a Specific Orientation by an Excimer Laser Annealing Method, Jpn. J. Appl. Phy., Vol. 32, pp 6190–6195, 1993.
Non-Patent Literature 3
K. Suzuki et al.; Correlation between Power Density Fluctuation and Grain Size Distribution of Laser Annealed Poly-Crystalline Silicon, SPIE Conference, Vol. 3618, pp. 310–319, 1999.